Filtering devices, for instance rotary disc filters, are used especially for filtering off particles from liquids.
A common rotary disc filter is disclosed, for instance, in SE-C-224 131 and the principle thereof is illustrated in FIG. 1. The rotary disc filter 1 comprises a horizontal, slowly rotating drum 11, which supports a number of parallel, disc-shaped filter elements 12 which are successively arranged in spaced-apart relationship along the centre axis of the drum and which extend radially away from the drum. The filter elements 12 have on their opposite lateral surfaces relatively large openings which are covered with a filter cloth 13. The drum has a number of openings through which the liquid A which is to be filtered can flow to the interior of the filter elements 12. The filtering liquid A is conducted through an inlet into the drum 11 and then falls down to the interior of the filter elements 12, from which the liquid A continues out through the filter cloth 13 so that the particles which are to be filtered off adhere to the inside of the filter cloth.
The rotary disc filter 1 can be cleaned by backwashing by means of a cleaning device of the type described, for instance, in SE-C-515 001 (WO 00/37159). This filter cleaning device is provided with spray nozzles 14 which are mounted on a plurality of fixed flush tubes 15, which project between the disc-shaped filter elements 12. The flush tubes 15 are adapted to conduct flushing liquid to the spray nozzles 14 and are connected to an elongate liquid-conducting tube 16 which extends parallel to the centre axis of the drum. The spray nozzles 14 spray liquid onto the filter cloth 13 axially from outside, in a direction opposite the filtering direction and flushed-off particles are collected in a discharge chute 17 which is placed inside the drum 11 in the upper portion thereof.
This known cleaning device may also be used in drum filters. An example of a prior art drum filter is shown in FIG. 6. The drum filter 101 has a drum 113, which is rotatably arranged about the centre axis C of the drum filter 101. The drum filter shown in FIG. 6 has a cleaning device 110 with spray nozzles arranged on a fixedly arranged flush tube.
The backwashing or cleaning of the rotary disc filter or drum filter occurs continuously or intermittently and, as a rule, at a relatively low pressure in the range of 6-8 bar.
Despite the backwashing, the filter cloth of rotary disc filters and drum filters has, however, a tendency to clog after some time of use due to deposition of sludge. The clogging of the filter cloth deteriorates the capacity and the degree of efficiency of the filter. As a result, supplementary cleaning is required after some time of use, for example once a day or at other intervals depending on the load to which the filter is subjected.
In some cases, the filter cloth may become “striped” due to insufficient cleaning of certain filter portions.
To increase the cleaning effect, chemical cleaning agents can be used in backwashing, but this is disadvantageous in terms of environment and working environment.
For the purpose of again increasing the filtering capacity of the filter after clogging, manual high pressure cleaning can be performed with a liquid that has a pressure of at least 50 bar. However, manual cleaning is difficult and time consuming. To allow manual cleaning, it is for practical reasons necessary to have a certain free space in the filter installation around each filter and sometimes also ramps and scaffolds so that a person who is to clean the filter cloth using, for example, a hand-held high pressure cleaning device reaches the filter and can walk around the filter. This makes the construction of such a rotary disc filter installation expensive and results in a need for a larger space.
WO 03/004131 discloses a cleaning device for rotary disc filters, which comprises a plurality of nozzles connected to a plurality of flush tubes and valves, in which each nozzle is intended for spraying fluid at a low or high pressure onto a disc-shaped filter element. The spray nozzles can be rotated in a swinging motion relative to the filter elements. A drawback of this cleaning device is that it requires a powerful and, thus, expensive high pressure pump to supply a fluid pressure that overcomes the losses in the many throttlings and the long tube lengths of the large number of tubes and nozzles for the fluid that is finally sprayed out of the nozzles to have a pressure that is sufficiently high for cleaning.
For satisfactory cleaning of the filter cloth, a plurality of high pressure spray nozzles are according to this prior art solution arranged very close to each other, which makes the filtering device considerably more expensive and complex. It would therefore be desirable to simplify the cleaning of rotary disc filters by high pressure and make such cleaning less expensive.
Moreover it would be desirable for reasons of environment and cost to reduce the consumption of cleaning liquid, chemicals and power. It would also be desirable to reduce the amount of peripheral equipment required for cleaning and to simplify the maintenance of the cleaning equipment by reducing the number of components of the same.